1. Field of the Invention
The present invention relates to a motor driving circuit for driving a video-camera focusing motor or the like.
2. Description of the Prior Art
Due to its relatively high control precision, a step motor is typically employed as a focusing motor for controlling the position of a focus lens in a video camera. Such step motor is driven generally by square waves. However, in driving a step motor by square waves, a problem arises in that the rotation of the motor may be rendered discontinuous with the occurrence of vibration and noise. In an attempt to absorb such vibration, an impedance roller or the like is usually employed. However, such means for eliminating or absorbing vibration may have limited results and may be an impediment to having a compact and lightweight structure.
In view of the above-described problems, the present inventor previously proposed that a step motor for effecting focus control be driven by sine waves. If a step motor is driven by sine waves, the rotation thereof is rendered relatively smooth thus reducing both vibration and noise, thereby eliminating the necessity of any mechanical countermeasure. As a result, a more compact and lightweight structure can be achieved. Furthermore, power consumption can be reduced by driving a step motor with sine waves.
When a step motor is driven by square waves, the motor may be reliably stopped at a detent position as, for example, illustrated in FIG. 14, in which the center of an excited pole 151A of a rotor 151 is opposed to the center of an excited pole 152A of a stator 152, however, on the other hand, when the step motor is driven by sine waves, the motor may not always be stopped at a detent position. If the step motor fails to stop at a detent position, a problem may arise in that, when the motor is started again, the rotation may be rendered nonsmooth which may cause an error.
The frequency of square waves may be changed by means of a frequency dividing counter or the like. However, in the case of sine waves, it is not easy to set a desired frequency.
Sine waves for driving a step motor can be produced by storing sine wave data in a read only memory (ROM), then supplying a predetermined clock signal to a counter to generate an address, and reading out the desired data from the ROM in accordance with the address obtained from the counter. As an example, consider the situation in which focus control is executed on the basis of the auto-focus control data received in every vertical period. In this situation, a step motor is driven by sine waves synchronized with the vertical period. Such sine waves can be generated by feeding horizontal sync pulses as a clock signal to a counter and then supplying the relevant address obtained from the counter to the ROM where the sine wave data are stored.
When a 2.sup.n counter is employed for generating an address in the manner described above, a desired frequency is settable by bit shifting. As an example, a 1-bit shift, corresponding to a count of 1, may be executed in a 2.sup.n counter with 2 clock pulses. The address to the ROM is then advanced stepwise at a speed of 1/2 so as to change the frequency to 1/2.
However, in the NTSC system for example, there are 525 horizontal sync pulses per frame. Since 525 is not a power product of 2, it is not impossible to employ a 2.sup.n counter for generating an address in this system.
In producing sine waves, it is customary in the prior art to store 1 frame of sine wave data in a ROM in a manner as previously mentioned. However, such storage of 1-frame of sine wave data increases the required capacity of the ROM.
Furthermore, to drive a step motor, multi-phase signals of mutually different phases are normally utilized. As a result, when forming two-phase sine waves for example, two groups of ROMs and address generators are utilized, thereby increasing the required amount of hardware.
Further, while a step motor typically requires a relatively large torque at the start of its rotation, such a large torque is not normally needed thereafter while the motor is rotating. Thus, it may be considered to increase the amplitude of the motor driving signal at the start of rotation and thereafter to decrease the amplitude so as to reduce the power consumption.
As previously described, when a step motor is driven by sine waves, the motor may not always stop exactly at a detent position. Further, if the amplitude of the motor driving signal is changed at any time other than when the motor is at a detent position, the motor rotation may not be smooth.